In the case of indirectly controlled fuel injectors, a so-called solenoid actuator controls the valve piston of a control valve and/or a servo valve, by means of which the pressure relationship between a control chamber and a valve chamber are influenced. The movement of the valve needle of the fuel injector is determined by the respective prevailing force relationships that are determined by means of a spring and also the pressures in the control chamber and in the valve chamber. These pressure relationships can be controlled by means of controlling the control valve.
FIG. 5 shows a schematic illustration of an indirectly controlled fuel injector 500 of this type. The fuel injector 500 comprises an outer casing 502 and also an inner casing 504. A displaceably mounted valve needle 510 is located within the inner casing 504 and said valve needle is prestressed by a spring 512. This spring urges the valve needle 510 downwards, so that in a starting state a discharge aperture 514 of the fuel injector 500 is closed.
A control drive 520 is located in an upper part of the fuel injector 500, which upper part is illustrated in a FIG. 5. The control drive 520 comprises a solenoid 522 that is located within an iron yoke 524. The control drive 520 further comprises a piston 530 or rather an armature 530 that is mounted in a displaceable manner and can be moved between a lower contact surface of the iron yoke 524 and a seat 532 of the control valve 520. The piston 530 is mechanically coupled by way of a coupling element 528 to a spring 526. The spring 526 is located within the solenoid 522.
The fuel injector 500 further comprises a control chamber 542 that is connected to a common rail system 550 by way of a high pressure line 540. A pressure sensor 552 is attached to the rail system and the pressure in the rail system can be monitored by means of said pressure sensor by a control unit that is not illustrated. The control chamber 542 is connected to a valve chamber 544 by way of a thin channel not illustrated in FIG. 5. Fuel can flow through this channel at a relatively slow flow rate. The control chamber 542 is moreover connected to a low pressure line 546 (a) by way of a line that is not provided with reference numerals and (b) by way of the control valve 520. The low pressure line is frequently also described as a leakage system 546.
If an injection operation is to be initiated, then the solenoid 522 is energized by means of applying a voltage U_solenoid. The solenoid 522 can be controlled by way of example by controlling the current. The current generates a magnetic force (described in FIG. 5 by F_solenoid) that acts on the piston 530 of the control valve 520. As soon as this magnetic force overcomes the force that is exerted by the spring 526 (described in FIG. 5 by F_spring), which force exerted by said spring fixes the control valve 520 in the non-energized case in the closed position, the piston 530 is moved in an accelerated manner in the direction of the solenoid 522 and/or the lower contact surface of the iron yoke 524. As a consequence, the control valve 520 opens and the highly pressurized fuel can discharge from the control chamber 542 into the low pressure line 546. The resulting pressure difference between the pressure in the 544 valve chamber and the pressure in the control chamber 542 then moves the valve needle 510 of the fuel injector 500 upwards in an accelerated manner and the discharge aperture 514 is opened.
If the injection operation is to be terminated, then the current flow through the solenoid 522 is interrupted. The magnetic force reduces and as soon as the magnetic force is less than the force of the spring 526, the valve piston of the control valve 520 is moved downwards in an accelerated manner into the closed position. The high pressure in the control chamber 542 is built back up and the valve needle 510 of the fuel injector 500 is moved downwards in an accelerated manner into the closed position.
The quantity of fuel to be injected consequently depends directly upon the control of the control valve 520. The dynamic behavior of the control valve 520 is influenced primarily by the opening process and the closing process. Tolerances in the opening behavior and the closing behavior of the control valve 520 lead directly to variations in the quantity of injected fuel.
The opening process is characterized by means of the time it takes for the solenoid 522 and the iron yoke 524 to build up force on the piston 530 and also the resilient force of the spring 526 that counteracts this build-up of force. The build-up of force is on the other hand determined by the geometric dimensions of the actuator (solenoid 522 and iron yoke 524), the electric parameters and/or the magnetic parameters of the solenoid 522 and also fundamentally by the energizing current and/or by the rates of change of the energizing current by means of the solenoid 522.
Different concepts are currently known for controlling (solenoid) fuel injectors. Generally, a difference is established between so-called high voltage concepts and low voltage concepts.
In the case of high voltage concepts, a stabilized voltage (so-called boost voltage in the range between typically 40 volt and 65 volt is provided by way of a costly circuitry in the control device. This voltage is then applied in the so-called boost phase to the fuel injector and guarantees a reproducible and highly dynamic build-up of force at the coil drive and/or solenoid actuator of the control valve of the indirectly controlled fuel injector.
In the case of low voltage concepts, it is only the battery voltage of the corresponding vehicle that is available to control the solenoid actuator. This has the advantage that costly circuitry is not required to generate the boost voltage and consequently considerable cost savings when manufacturing injection systems can be achieved. However, the disadvantage of low voltage concepts resides in the fact that the battery voltage in the vehicle can fluctuate over a relatively wide range of typically 6 volt to 19 volt depending upon operating conditions. This has the result that the build-up of force at the coil drive and/or solenoid actuator is influenced by the prevailing operating voltage. However, the build-up of force at the actuator is the determining variable of the valve dynamics of the control valve. Consequently, the opening behavior of the servo valve and as a consequence also the injection rate depend directly upon the prevailing voltage at the injector.
FIG. 6 illustrates for an indirectly controlled fuel injector:                (a) the progression with respect to time of the voltage that is applied at the coil drive,        (b) the progression with respect to time of the coil current that is flowing through the coil drive and        (c) the progression with respect to time of the injection rate.        
The continuous lines result from measurements in which the available battery voltage amounts to 19 volt.
The broken lines result from measurements in which the available battery voltage amounts to 9 volt.
The quantity of fuel that is injected overall during an injection operation is determined from the integral with respect to time of the injection rate and is consequently significantly dependent upon the injection rate and its progression with respect to time. The point in time in which the injection commences not only influences the injection process but also in particular control ranges it influences the maximum rate that can be achieved during the injection process.
As described above, a quick build-up of force results in the control valve opening quickly and consequently also in the nozzle needle of the indirectly controlled fuel injector opening quickly. A quick build-up of force is facilitated by means of the coil drive of the control valve by virtue of a high (battery) voltage and finally by virtue of a high current strength.
These considerations consequently explain the difference between the two graph progressions that are illustrated in the lower image of FIG. 6. When a higher battery voltage is available, the control valve opens more quickly and the resulting injection operation commences (after a particular hydraulic delay) earlier. As a consequence, when the battery voltage is higher, a higher value is produced for the integral with respect to time of the injection rate and the overall injected quantity of fuel per injection pulse is greater than when a lower battery voltage is available.